In the first release of Long Term Evolution (LTE) standard, a cell-specific reference signal (CRS) was introduced. It is the most basic downlink reference signal transmitted in every downlink sub-frame and in every resource block in the frequency domain. The cell-specific reference signal can thus cover the entire bandwidth of the cell. The cell-specific reference signal is used by communication devices for performing channel estimation in order to obtain coherent demodulation of downlink physical channels and for performing cell selection and making handover decisions. For communication devices adopting release 8 and 9 of LTE, the cell-specific reference signal is also used to acquire channel-state information (CSI).
In LTE Release 10, support for channel-state information reference signals (CSI-RS) is introduced. A reason for introducing CSI-RS is to improve channel estimation for coherent demodulation even under the most extreme channel conditions including very fast channel variations in both time and frequency domain without introducing much overhead. Furthermore, it is believed that introducing a new type of reference signal only targeting CSI entails a flexible and in general lower time/frequency density and corresponding lower overhead per reference signal. CSI-RSs are intended for communication devices to acquire channel-state information.
A transmission hypothesis in multiple-input, multiple-output (MIMO) or Coordinated Multipoint (CoMP) comprises a signal hypothesis and an interference hypothesis. The signal hypothesis specifies transmission points from which data is assumed to have been transmitted, and the interference hypothesis estimates interference suffered during the assumed data transmission. The most desirable would be for the communication device to report CSI of all possible transmission hypotheses, but the feedback overhead and complexity of communication device implementation is proportional to the number of hypotheses and makes this hard to achieve. CSI corresponding to one transmission hypothesis is defined as a CSI process. The CSI process is determined by the association of a signal hypothesis and an interference hypothesis, where the signal hypothesis and interference hypothesis are measured through CSI-RS and CSI interference measurement (IM) resource, respectively.
The configuration of CSI resources is done in a radio resource control (RRC) configuration. For example, for Transmission Mode 9 (TM9), two parameters need to be addressed in the RRC configuration to configure CSI: CSI-RS and zeroTxPower CSI-RS [refer to 3GPP TS 36.331 V11.3.0]. It is possible to configure one CSI-RS per cell/sector. Basically, in order to measure the channel quality of CSI-RS of one cell/sector accurately, the neighboring cells/sectors' CSI-RSs have to be muted, i.e. configured with zero transmit power (ZP, or zeroTxPower) CSI-RS. For TM10, CSI-IM is introduced to measure the interference from the neighboring cell/sectors. The resource used for CSI-IM also needs to be configured. When the resource is configured for interference measurement, the CSI-RS in that resource element has to be muted.
Furthermore, while only one CSI report is supported in TM9, it is possible to have up to 4 CSI processes for TM10 and each CSI process keeps track of channel quality measurement and generates one CSI report.
Difficulties arise in case the network deployment is complex, involving a lot of interference hypotheses, while the CSI resources are limited. A main challenge will be how to configure CSI-RS and CSI-IM resources in an efficient and simple way.
One example of the prior art solution of CSI-RS/CSI-IM configuration is shown in FIGS. 1a, 1b, 1c. In this example, a deployment with three sectors is used. Sector 1, sector 2 and sector 3 are configured with CSI-RS resource configuration 15, 16 and 17 respectively. There are, in this example, three CSI processes per communication device (as illustrated in FIG. 1a) and all CSI processes of a communication device are configured with the same CSI-RS that is associated with the serving sector (i.e. the CSI-RS of the serving sector is used also for the other sectors). For example, sector 1 is configured with CSI-RS configuration 15, and a communication device having sector 1 as serving sector is configured with this CSI-RS configuration for all its three CSI processes (CSI processes 1, 2 and 3). The communication device thus sends, to a network node, a respective CSI-report (see FIG. 1a) corresponding to a respective CSI process.
Thus, in the scenario of FIGS. 1a, 1b, 1c, each sector is sending on one CSI-RS resource and is quiet in nine. For example, Sector 3 transmits on CSI resource element 17 (indicated by non-zero power, NZP, see FIG. 1b) and is quiet in CSI resource elements 5, 6, 9, 10, 13, 14, 15, 16, 18 (indicated by zero-power, ZP, see e.g. FIG. 1a). Different CSI-IM configurations are configured for different CSI processes to measure the different interference hypotheses; in the scenario of FIGS. 1a, 1b, 1c, three CSI-IM hypotheses are tested in each sector. For example (see e.g. FIG. 1b), in Sector 1, interference hypotheses CSI-IM1, CSI-IM2 and CSI-IM3 are tested. When the configuration is used for CSI-IM, the CSI-RS and data transmission is muted in the serving sector (see e.g. FIG. 1c). However, the transmissions in the neighboring sectors are not affected, i.e. they can be muted or they can transmit any information, such as CSI-RS or data.
A first drawback of this solution is the high overhead waste. Overall, there are in total 9 over 144 resource elements to be muted for each sector. The overhead caused by the zero power CSI resource elements is thus about 6.25%.
A second drawback is that all configurations and interference hypotheses are not supported. As seen in FIG. 1c, only 6 hypotheses assuming that the transmissions come from a single transmission point with 6 interference scenarios are supported. Other configurations such as for example joint transmission, where the signal can be transmitted from multiple sectors, are not supported.
Furthermore, the known solution is complicated in the sense that it is not scalable, meaning that whenever there is new sector or the situation changed, the configurations for all sectors need to be changed.